In the past, it has been common to extrude hygroscopic, synthetic resins, such as ABS (acrylonitrile-butadiene-styrene resins), into finished products. However, such extrusion has in the past been preceded by a dehumidifying procedure in which the hygroscopic resins are pre-dried for several hours. When the dehumidified resins are extruded through a single vented extruder, a finished product substantially free of pock marks, dimples, streaks, surface roughness, and poor gloss results. A good dehumidification system is not only expensive but also requires a substantial electrical power supply. In addition, the systems occupy valuable floor area in manufacturing operations.
When the hygroscopic resinous material is dehumidified in a hopper dryer, the temperature must be carefully controlled to prevent overheating. Even with careful temperature control, experience has shown an occasional batch of material will become overheated and fuse together causing both loss of time and waste of material.
Another frequent problem with hopper dryers is the difficulty in maintaining uniformly dehumidified material for subsequent use in an extruder. This problem results from the non-homogeneity of moisture content in hygroscopic particulate material.
Previous combinations of a dryer and an extruder have inadequately accommodated the need for rapid color changes often encountered in production operations. The inadequacy results directly from the lead time required to dry a differently colored stock of hygroscopic material for extrusion.
Some rotary screw extruders in the past have employed reduced pressure vent sections to remove volatiles from the extrudate. Typically, however, these vented sections are pressure sealed from one another such as the vented sections of U.S. Pat. No. 2,992,679 which issued to W. W. Twaddle. But, such sealing between sections reduces the effective devolatilization length and is ineffective for use to extrude hygroscopic synthetic resinous materials.
Other extruders have employed continuous devolatilization sections in the extruder screw such as disclosed by U.S. Pat. No. 3,524,222 to Gregory et al. This type of extruder results in nonuniform flow rates which are unsuitable for the extrusion of high quality finished products. Nonuniform flow rates are aggravated by pressure gradients which often occur in long vent sections. Moreover, the extrudate in such extruders frequently develops thermal nonuniformity which is induced by volatile substances undergoing a liquid-vapor phase change. Since viscosity is highly dependent on temperature, such thermal non-uniformity also has an adverse effect on extruded products.
Currently the PET extrusion process uses resin in powder form. The extruder screw needed to process the powder is a two-stage type which means that it requires a special extraction section for vapor and volatile removal to a vacuum source. Without the extraction, bubbles will appear in the product.
Other producers of PET use resin in the form of pellets. This allows a barrier type of screw design because the porosity of the pellets permits vapor and volatile materials to escape through the pellets to the feed hopper counter to the direction of polymer flow. The barrier screw is known to be more efficient, more stable, more versatile, and have greater potential rate than the standard two-stage screw used for PET powder which is comprised of five separate sections: 1) feed; 2) compression; 3) metering; 4) extraction; and 5) pumping. However, PET powder in a conventional barrier screw is compacted so that vapor can not escape through the solid feed as it does for pellet feed.
In general, volatile and vapor extraction is commonly done during extrusion for single (and twin-screw) types of extruders. Most common is the venting through a port in the barrel wall such as described in U.S. Pat. Nos. 3,992,500 and 3,593,843. However, this has the restriction that the vent location is fixed. Venting through the screw is an alternative that allows positioning the vent optimally for each screw.
Screw venting has been accomplished through a bore in the screw to a vent as described in U.S. Pat. Nos. 4,478,519 and 2,774,105 or vents as described in U.S. Pat. No. 3,986,709 in the melt channel. In each case a voided volume is created in the melt flow to allow gases to accumulate and be removed via the vent(s) and screw bore. In the first device, centrifugal force is used to separate the gases from the extrudate. In the second device, walls in the flow channel divert the polymer to accomplish a voided volume for the vents. In the third example, a deep channel section for the melt is provided that will only partially fill at the prevailing rate. Venting is then provided in the deep channel section (extraction section). This third example is typical of current art for PET powder processing. For present purposes, extra volume is required for extraction of gases, which requires extra screw length at the expense of rate and efficiency.